1. Field of the Invention
This invention relates to an optical scanning apparatus and an image forming apparatus using the same, and particularly is suitable for an apparatus such as a laser beam printer or a digital copier having, for example, the electrophotographic process adapted to reflect and deflect one or more beams emitted from light source means by a polygon mirror as a light deflector, and optically scan a surface to be scanned through an fxcex8 lens system having the fxcex8 characteristic to thereby record image information.
Particularly, the present invention relates to an optical scanning apparatus in which the shapes of a plurality of lenses constituting scanning optical means are appropriately set to thereby correct the curvature of image field in the main scanning direction and the sub scanning direction, the fxcex8 characteristic and the fluctuation of the magnification in the sub scanning direction so that good images are always obtained, and an image forming apparatus using the same.
2. Related Background Art
Heretofore, in an optical scanning apparatus such as a laser beam printer, a beam light-modulated and emitted from light source means in conformity with an image signal is periodically deflected by a light deflector comprising, for example, a rotatable polygon mirror, and is converged into a spot shape on the surface of a photosensitive recording medium (photosensitive drum) by an fxcex8 lens system having the fxcex8 characteristic, and optically scans the surface of the recording medium to thereby effect image recording.
FIG. 18 of the accompanying drawings is a schematic view of the essential portions of an optical scanning optical system according to the prior art. In this figure, a divergent beam emitted from light source means 91 is made into a substantially parallel beam by a collimator lens 92, and this beam (the quantity of light) is shaped by an aperture stop 93 and enters a cylindrical lens 94 having refractive power only in the sub scanning direction. Of the beam having entered the cylindrical lens 94, that part in the main scanning section intactly emerges and that part in the sub scanning section converges and is imaged as a substantially linear image near the deflecting surface 95a of a light deflector 95 comprising a rotatable polygon mirror.
The beam reflected and deflected by the deflecting surface 95a of the light deflector 95 is directed onto the surface of a photosensitive drum as a surface 97 to be scanned through an fxcex8 lens system 96 having the fxcex8 characteristic, and the light deflector 95 is rotated in the direction of arrow A to thereby optically scan the surface 97 of the photosensitive drum in the direction of arrow B (the main scanning direction) and effect the recording of image information.
To effect highly accurate recording of image information in an optical scanning apparatus of this kind, it is important that curvature of image field is well corrected over the entire surface to be scanned and the spot diameter is uniform, that an equal speed is kept when the surface of the photosensitive drum is light-scanned (fxcex8 characteristic), that the lateral magnification in the sub scanning direction is uniformly corrected over the entire effective scanning area and the spot diameter in the sub scanning direction is uniform, and that in a multibeam scanning apparatus using light source means emitting a plurality of beams, the lateral magnification in the sub scanning direction is uniformly corrected over the entire effective scanning area and the pitch interval between scanning lines is made constant. Various optical scanning apparatuses or fxcex8 lens systems satisfying such optical characteristics have heretofore been proposed.
For example, Japanese Patent Application Laid-Open No. 7-318796 discloses an fxcex8 lens system comprising a combination of a glass toric lens and a plastic toric lens each having a cylindrical lens surface on the incidence surface side thereof and a toric surface on the emergence surface side thereof. In this publication, however, one surface is a cylindrical surface and therefore, there has been the problem that the degree of freedom is small with regard to the above-mentioned aberration correction and the above-mentioned aberration correction is difficult. So, in the present invention, as will be described later, all fxcex8 lenses constituting an fxcex8 lens system are made into toric lenses having toric surfaces on both surfaces thereof. Further, each of the aforementioned fxcex8 lenses has its both surfaces made into a non-arcuate main scanning sectional shape and has its radius of curvature in the sub scanning direction continuously varied, whereby the above-mentioned aberrations are corrected well. Also, the above-mentioned publication does not bear the description of sub scanning magnification, and has not taken it into consideration to reduce the degree of sensitivity of focus fluctuation in the sub scanning direction and to uniformly correct sub scanning magnification in an effective scanning area on a surface to be scanned. The present invention takes these into consideration and can construct an optical scanning apparatus suited for the highly accurate recording of image information.
Also, in Embodiment 1 of Japanese Patent Application Laid-Open No. 7-318796, the power of a glass toric lens 22 on the scanned surface 14 side in the main scanning direction is greater than the power of a plastic toric lens 21 on the polygon mirror 12 side in the main scanning direction and therefore, a problem is left in achieving compactness. In Embodiment 2 of Japanese Patent Application Laid-Open No. 7-318796, both of the power of the plastic toric lens 21 in the sub scanning direction and the power of the glass toric lens 22 in the sub scanning direction are positive and therefore, there is left the problem that when the two lenses 21 and 22 are brought close to the polygon mirror 12 side, sub scanning magnification increases.
On the other hand, with the compactness and lower cost of laser beam printers, digital copiers, etc., similar conditions have also been required for image forming apparatuses.
What makes these requirements compatible is proposed, for example, in Japanese Patent Application Laid-Open No. 10-232346. In this publication, curvature of image field and distortion are corrected well and the influence of a change or the like in the spot diameter by image height is reduced.
However, to achieve further compactness of the optical scanning apparatus, it is necessary to shorten the focal length of the fxcex8 lens system, widen the angle of field thereof and bring the fxcex8 lens close to the polygon mirror which is deflecting means. All these are factors which make aberration correction difficult, and there has been the problem that when compactness has been made, the curvature of image field in a wide field angle area, the fxcex8 characteristic, and the fluctuation of the magnification in the sub scanning direction are not corrected well.
Also, another problem arises with the widening of the angle of field. Heretofore, a beam emitted from light source means has been incident on the deflecting surface of the polygon mirror obliquely with respect to the optical axis of the fxcex8 lens system, and at this time, the reflecting position at which the beam is reflected by the deflecting surface changes continuously and asymmetrically with respect to the center of scanning. This asymmetrical change in the reflecting position affects particularly the imaging position and it becomes difficult to obtain flat curvature of image field.
The above-mentioned asymmetrical change in the reflecting position is caused by making the beam from the light source means incident obliquely with respect to the optical axis of the fxcex8 lens system and therefore, it can be eliminated by making the beam from the light source means incident from the direction of the optical axis of the fxcex8 lens system, but the disposition is unreasonable and the beam must be made incident from the outside of the fxcex8 lens system and therefore, the asymmetry given to the curvature of image field by the asymmetrical change in the reflecting position cannot be eliminated.
So, for example, Japanese Patent Application Laid-Open No. 4-60608 and Japanese Patent Application Laid-Open No. 9-265041 disclose various examples in which vertical asymmetry is introduced in to the meridional shape of the fxcex8 lens constituting the fxcex8 lens system.
However, to achieve the compactness of the fxcex8 lens system, curvature of image field, the fxcex8 characteristic and the fluctuation of the magnification in the sub scanning direction must be corrected well even in a wide field angle area exceeding the angle of field xc2x147xc2x0, and these have not always been satisfactory.
Also, to make the optical scanning apparatus correspond to a multibeam, it has been necessary to make a beam emergent from a collimator lens into a substantially parallel beam in order to reduce the jitter in the main scanning direction.
Also, Japanese Patent Application Laid-Open No. 10-333069 discloses a construction in which in order to solve the problem arising in a multibeam scanning optical system that the relative interval between scanning lines is changed by the scanning position, the power distribution of a scanning lens and a correction lens in the sub scanning direction is designed such that the effect of correcting the curvature of image field in the sub scanning direction is obtained. However, the power of the correction lens nearest to the surface to the scanned in the main scanning direction is greatest and therefore, a problem is left in achieving compactness.
Also, in Japanese Patent Application Laid-Open No. 5-5852, in order to realize a bright fxcex8 lens system, the fxcex8 lens system is of two-unit two-lens construction, and prescribes the relations among the sub scanning magnification xcex2, the composite focal length fs with respect to the sub scanning direction and the radii of curvature ry3 and ry4 of the third and fourth surfaces in the sub scanning direction. However, the power of the first lens near to a rotatable polygon mirror in the main scanning direction is smaller than the power of the second lens near to a surface to be scanned in the main scanning direction and therefore, a problem is left in achieving compactness.
The present invention has as its object the provision of an optical scanning apparatus in which the shapes of a plurality of lenses constituting scanning optical means are appropriately set to thereby well correct the curvature of image field and distortion in the main scanning direction and the curvature of image field and the fluctuation of magnification in the sub scanning direction and which, in spite of a compact construction, is suited for highly definite printing in which the sub scanning magnification is restrained low, and an image forming apparatus using the same.
To achieve the above object, the optical scanning apparatus of the present invention is an optical scanning apparatus comprising incidence optical means for causing a beam emitted from light source means to be incident on deflecting means, and scanning optical means for causing the beam reflected and deflected by the deflecting means to be imaged on a surface to be scanned, characterized in that the scanning optical means has a first lens on the deflecting means side and a second lens on the surface to be scanned side, the first lens has positive power in the main scanning direction and has negative power in the sub scanning direction, the power of the first lens in the main scanning direction is greater than the power of the second lens in the main scanning direction, and the second lens has positive power in the sub scanning direction.
In the above-described optical scanning apparatus, when the power of the scanning optical means in the main scanning direction is defined as xcfx86m and the power of the second lens in the main scanning direction is defined as xcfx862m, the condition that
xe2x88x920.5xe2x89xa6xcfx862m/xcfx86mxe2x89xa60.15 
is satisfied.
In the above-described optical scanning apparatus, the first lens and the second lens are disposed more adjacent to the deflecting means than to the midpoint of the distance from the deflecting surface of the deflecting means to the surface to be scanned.
In the above-described optical scanning apparatus, each of the first and second lenses is an aspherical lens in which the main scanning sectional shapes of both surfaces are non-arcuate shapes.
In the above-described optical scanning apparatus, the scanning optical means has a plurality of meridional asymmetrical surfaces of which the main scanning sectional shape varies asymmetrically in the main scanning direction across an optical axis.
In the above-described optical scanning apparatus, the meridional asymmetrical surfaces are formed on that lens surface of the second lens which faces the surface to be scanned.
In the above-described optical scanning apparatus, each of the first and second lenses is an aspherical lens in which the radii of curvature of both surfaces in the sub scanning direction continuously vary away from an optical axis along the main scanning direction on at least one side across the optical axis.
In the above-described optical scanning apparatus, the scanning optical means has a plurality of sagittal asymmetrically varying surfaces of which the radii of curvature in the sub scanning direction vary asymmetrically in the main scanning direction across an optical axis.
In the above-described optical scanning apparatus, two or more of the plurality of sagittal asymmetrically varying surfaces are sagittal deformed surfaces of which the sizes of the radii of curvature in the sub scanning direction differ in the main scanning direction across the optical axis, and on two or more of the two or more sagittal deformed surfaces, the sides on which the radii of curvature in the sub scanning direction become larger than the radii of curvature on the optical axis are all present on the same side with respect to the optical axis, or the sides on which the radii of curvature in the sub scanning direction become smaller than the radii of curvature on the optical axis are all present on the same side with respect to the optical axis.
In the above-described optical scanning apparatus, the scanning optical means has a plurality of main and sub asymmetrical surfaces which are the meridional asymmetrical surfaces and also are the sagittal asymmetrically varying surfaces.
In the above-described optical scanning apparatus, when the power of the first lens in the main scanning direction is defined as xcfx861m and the power of the scanning optical means in the main scanning direction is defined as xcfx86m, the condition that
0.85xe2x89xa6xcfx861m/xcfx86mxe2x89xa61.3 
is satisfied.
In the above-described optical scanning apparatus, when the power of the first lens in the sub scanning direction is defined as xcfx861s and the power of the second lens in the sub scanning direction is defined as xcfx862s, the condition that
xe2x88x921.8xe2x89xa6xcfx861s/xcfx862sxe2x89xa6xe2x88x920.4 
is satisfied.
In the above-described optical scanning apparatus, when the power of the first lens in the main scanning direction is defined as xcfx861m and the power of the first lens in the sub scanning direction is defined as xcfx861s and the power of the second lens in the main scanning direction is defined as xcfx862m and the power of the second lens in the sub scanning direction is defined as xcfx862s, the condition that
xcfx861s less than xcfx862m less than xcfx861m less than xcfx862s 
is satisfied.
In the above-described optical scanning apparatus, when the power of the first lens in the main scanning direction is defined as xcfx861m and the power of the first lens in the sub scanning direction is defined as xcfx861s and the power of the second lens in the main scanning direction is defined as xcfx862m and the power of the second lens in the sub scanning direction is defined as xcfx862s, the condition that
|xcfx862m| less than |xcfx861m| less than |xcfx861s| less than |xcfx862s|
is satisfied.
In the above-described optical scanning apparatus, when the radius of curvature of that surface of the first lens which faces the deflecting means in the main scanning direction is defined as R1m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R1s and the radius of curvature of that surface of the first lens which faces the surface to be scanned in the main scanning direction is defined as R2m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R2s and the radius of curvature of that surface of the second lens which faces the deflecting means in the main scanning direction is defined as R3m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R3s and the radius of curvature of that surface of the second lens which faces the surface to be scanned in the main scanning direction is defined as R4m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R4s, the conditions that
xe2x80x83R1m less than R2m less than 0 less than R4m less than R3m
R2s less than R1s less than 0 
R3s less than R4s less than 0 
R1m less than R1s less than 0 
R2s less than R2m less than 0 
|R4s| less than R4m 
are satisfied.
In the above-described optical scanning apparatus, the effective end portions of all surfaces of the first and second lenses are displaced more toward the deflecting means than the surface vertexes of the lenses.
In the above-described optical scanning apparatus, when the fxcex8 coefficient of the scanning optical means is defined as k and the effective scanning width thereof is defined as W, the condition that
k/Wxe2x89xa60.6 
is satisfied.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to the surface to be scanned is defined as L and the effective scanning width is defined as W, the condition that
L/Wxe2x89xa60.8 
is satisfied.
In the above-described optical scanning apparatus, when the distance from the defecting surface of the deflecting means to that surface of the second lens which faces the surface to be scanned is defined as d and the effective scanning width is defined as W, the condition that
d/Wxe2x89xa60.2 
is satisfied.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to that surface of the second lens which faces the surface to be scanned is defined as d and the distance from the deflecting surface of the deflecting means to the surface to be scanned is defined as L, the condition that
d/Lxe2x89xa60.25 
is satisfied.
In the above-described optical scanning apparatus, each of the first and second lenses comprises a toric lens of which both surfaces have toric surfaces.
In the above-described optical scanning apparatus, the first lens is a meniscus lens of which the shape in the main scanning direction has its convex surface facing the surface to be scanned.
In the above-described optical scanning apparatus, the incidence optical means has a condensing lens for converting the beam emitted from the light source means into a substantially parallel beam.
In the above-described optical scanning apparatus, a plurality of beams are emitted from the light source means.
In the above-described optical scanning apparatus, the incidence optical means has a condensing lens for converting the beam emitted from the light source means into a convergent beam.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to the converging point by the condensing lens is defined as S and the fxcex8 coefficient of the scanning optical means is defined as k, the condition that
|S|/kxe2x89xa75 
is satisfied.
Also, the image forming apparatus of the present invention is an image forming apparatus provided with the above-described optical scanning apparatus, a photosensitive member disposed on the surface to be scanned, a developing device for developing an electrostatic latent image formed on the photosensitive member by the beam scanned by the optical scanning apparatus as a toner image, a transferring device for transferring the developed toner image to a transfer material, and a fixing device for fixing the transferred toner image on the transfer material.
Also, the image forming apparatus of the present invention is an image forming apparatus provided with the above-described optical scanning apparatus, and a printer controller for converting code data inputted from an external device into an image signal and inputting it to the optical scanning apparatus.
Also, the optical scanning apparatus of the present invention is an optical scanning apparatus comprising incidence optical means for causing a beam emitted from light source means to be incident on deflecting means, and scanning optical means for causing the beam reflected and deflected by the deflecting means to be imaged on a surface to be scanned, characterized in that the scanning optical means has a first lens on the deflecting means side, and has a second lens on the surface to be scanned side, each of the first and second lenses comprises a toric lens of which both surfaces have toric surfaces, the first lens has negative power in the sub scanning direction, and the second lens has positive power in the sub scanning direction.
In the above-described optical scanning apparatus, when the power of the scanning optical means in the main scanning direction is defined as xcfx86m and the power of the second lens in the main scanning direction is defined as xcfx862m, the condition that
xe2x88x920.5xe2x89xa6xcfx862m/xcfx86mxe2x89xa60.15 
is satisfied.
In the above-described optical scanning apparatus, the first lens and the second lens are disposed more adjacent to the deflecting means than to the midpoint of the distance from the deflecting surface of the deflecting means to the surface to be scanned.
In the above-described optical scanning apparatus, each of the first and second lenses is an aspherical lens in which the main scanning sectional shapes of both surfaces are non-arcuate shapes.
In the above-described optical scanning apparatus, the scanning optical means has a plurality of meridional asymmetrical surfaces of which the main scanning sectional shapes vary asymmetrically in the main scanning direction across an optical axis.
In the above-described optical scanning apparatus, the meridional asymmetrical surfaces are formed on that lens surface of the second lens which faces the surface to be scanned.
In the above-described optical scanning apparatus, each of the first and second lenses is an aspherical lens in which the radii of curvature of both surfaces in the sub scanning direction continuously vary away from an optical axis along the main scanning direction on at least one side across the optical axis.
In the above-described optical scanning apparatus, the scanning optical means has a plurality of sagittal asymmetrically varying surfaces of which the radii of curvature in the sub scanning direction vary asymmetrically in the main scanning direction across an optical axis.
In the above-described optical scanning apparatus, two or more of the plurality of sagittal asymmetrically varying surfaces are sagittal deformed surfaces of which the sizes of the radii of curvature in the sub scanning direction differ in the main scanning direction across the optical axis, and on two or more of the two or more sagittal deformed surfaces, the sides on which the radii of curvature in the sub scanning direction become larger than the radii of curvature on the optical axis are all present on the same side with respect to the optical axis, or the sides on which the radii of curvature in the such scanning direction become smaller than the radii of curvature on the optical axis are all present on the same side with respect to the optical axis.
In the above-described optical scanning apparatus, the scanning optical means has a plurality of main and sub asymmetrical surfaces which are the meridional asymmetrical surfaces and also are the sagittal asymmetrically varying surfaces.
In the above-described optical scanning apparatus, when the power of the first lens in the main scanning direction is defined as xcfx861m and the power of the scanning optical means in the main scanning direction is defined as xcfx86m, the condition that
xe2x80x830.85xe2x89xa6xcfx861m/xcfx86mxe2x89xa61.3
is satisfied.
In the above-described optical scanning apparatus, when the power of the first lens in the sub scanning direction is defined as xcfx861s and the power of the second lens in the sub scanning direction is defined as xcfx862s, the condition that
xe2x88x920.8xe2x89xa6xcfx861s/xcfx862sxe2x89xa6xe2x88x920.4 
is satisfied.
In the above-described optical scanning apparatus, when the power of the first lens in the main scanning direction is defined as xcfx861m and the power of the first lens in the sub scanning direction is defined as xcfx861s and the power of the second lens in the main scanning direction is defined as xcfx862m and the power of the second lens in the sub scanning direction is defined as xcfx862s, the condition that
xcfx861s less than xcfx862m less than xcfx861m less than xcfx862s 
is satisfied.
In the above-described optical scanning apparatus, when the power of the first lens in the main scanning direction is defined as xcfx861m and the power of the first lens in the sub scanning direction is defined as xcfx861s and the power of the second lens in the main scanning direction is defined as xcfx862m and the power of the second lens in the sub scanning direction is defined as xcfx862s, the condition that
xe2x80x83|xcfx862m| less than |xcfx861m| less than |xcfx861s| less than |xcfx862s|
is satisfied.
In the above-described optical scanning apparatus, when the radius of curvature of that surface of the first lens which faces the deflecting means in the main scanning direction is defined as R1m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R/S and the radius of curvature of that surface of the first lens which faces the surface to be scanned in the main scanning direction is defined as R2m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R2s and the radius of curvature of that surface of the second lens which faces the deflecting means in the main scanning direction is defined as R3m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R3s and the radius of curvature of that surface of the second lens which faces the surface to be scanned in the main scanning direction is defined as R4m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R4s, the conditions that
R1m less than R2m less than 0 less than R4m less than R3m 
R2s less than R1s less than 0 
R3s less than R4s less than 0 
R1m less than R1s less than 0 
xe2x80x83R2s less than R2m less than 0
|R4s| less than R4m 
are satisfied.
In the above-described optical scanning apparatus, the effective end portions of all surfaces of the first and second lenses are displaced more toward the deflecting means than the surface vertexes of the lenses.
In the above-described optical scanning apparatus, when the fxcex8 coefficient of the scanning optical means is defined as k and the effective scanning width thereof is defined as W, the condition that
k/Wxe2x89xa60.6 
is satisfied.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to the surface to be scanned is defined as L and the effective scanning width is defined as W, the condition that
L/Wxe2x89xa60.8 
is satisfied.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to that surface of the second lens which faces the surface to be scanned is defined as d and the effective scanning width is defined as W, the condition that
xe2x80x83d/Wxe2x89xa60.2
is satisfied.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to that surface of the second lens which faces the surface to be scanned is defined as d and the distance from the deflecting surface of the deflecting means to the surface to be scanned is defined as L, the condition that
d/Lxe2x89xa60.25 
is satisfied.
In the above-described optical scanning apparatus, the first lens is a meniscus lens of which the shape in the main scanning direction has its convex surface facing the surface to be scanned.
In the above-described optical scanning apparatus, the incidence optical means has a condensing lens for converting the beam emitted from the light source means into a substantially parallel beam.
In the above-described optical scanning apparatus, a plurality of beams are emitted from the light source means.
In the above-described optical scanning apparatus, the incidence optical means has a condensing lens for converting the beam emitted from the light source means into a convergent beam.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to the converging point by the condensing lens is defined as S and the fxcex8 coefficient of the scanning optical means is defined as k, the condition that
|S|/kxe2x89xa75 
is satisfied.
Also, the image forming apparatus of the present invention is an image forming apparatus provided with the above-described optical scanning apparatus, a photosensitive member disposed on the surface to be scanned, a developing device for developing an electrostatic latent image formed on the photosensitive member by the beam scanned by the optical scanning apparatus as a toner image, a transferring device for transferring the developed toner image to a transfer material, and a fixing device for fixing the transferred toner image on the transfer material.
Also, the image forming apparatus of the present invention is an image forming apparatus provided with the above-described optical scanning apparatus, and a printer controller for converting code data inputted from an external device into an image signal and inputting it to the optical scanning apparatus.
Also, the optical scanning apparatus of the present invention is an optical scanning apparatus comprising incidence optical means for causing a beam emitted from light source means to be incident on deflecting means, and scanning optical means for causing the beam reflected and deflected by the deflecting means to be imaged on a surface to be scanned, characterized in that the scanning optical means has two or more optical elements including a first optical element nearest to the deflecting means and a second optical element nearest to the surface to be scanned, the first optical element has positive power in the main scanning direction, and has negative power in the sub scanning direction, the second optical element has positive power in the sub scanning direction, and among the two or more optical elements included in the scanning optical means, the power of the first optical element in the main scanning direction is greatest.
In the above-described optical scanning apparatus, when the power of the first optical element in the main scanning direction is defined as xcfx861m and the power thereof in the sub scanning direction is defined as xcfx861s and the power of the second optical element in the main scanning direction is defined as xcfx862m and the power thereof in the sub scanning direction is defined as xcfx862s, the condition that
xcfx861s less than xcfx862m less than xcfx861m less than xcfx862s 
is satisfied.
In the above-described optical scanning apparatus, when the power of the first optical element in the main scanning direction is defined as xcfx861m and the power thereof in the sub scanning direction is defined as xcfx861s and the power of the second optical element in the main scanning direction is defined as xcfx862m and the power thereof in the sub scanning direction is defined as xcfx862s, the condition that
|xcfx862m| less than |xcfx861m| less than |xcfx861s| less than |xcfx862s|
is satisfied.
In the above-described optical scanning apparatus, the first optical element is a meniscus lens of which the shape in the main scanning direction has its convex surface facing the surface to be scanned.
In the above-described optical scanning apparatus, when the fxcex8 coefficient of the scanning optical means is defined as k and the effective scanning width thereof is defined as W, the condition that
k/Wxe2x89xa60.6 
is satisfied.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to the surface to be scanned is defined as L and the effective scanning width is defined as W, the condition that
L/Wxe2x89xa60.8 
is satisfied.
In the above-described optical scanning apparatus, the first optical element and the second optical element are disposed more adjacent to the deflecting means than to the midpoint of the distance from the deflecting surface of the deflecting means to the surface to be scanned.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to that surface of the second optical element which faces the surface to be scanned is defined as d and the effective scanning width is defined as W, the condition that
d/Wxe2x89xa60.2 
is satisfied.
In the above-described optical scanning apparatus, when the distance from the deflecting surface of the deflecting means to that surface of the second optical element which faces the surface to be scanned is defined as d and the distance from the deflecting surface of the deflecting means to the surface to be scanned is defined as L, the condition that
d/Lxe2x89xa60.25 
is satisfied.
In the above-described optical scanning apparatus, the first optical element or the second optical element is a lens.
In the above-described optical scanning apparatus, the first optical element or the second optical element is a reflecting mirror.
In the above-described optical scanning apparatus, the first optical element and the second optical element are lenses.
In the above-described optical scanning apparatus, when the radius of curvature of that surface of the first lens which faces the deflecting means in the main scanning direction is defined as R1m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R1s and the radius of curvature of that surface of the first lens which faces the surface to be scanned in the main scanning direction is defined as R2m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R2s and the radius of curvature of that surface of the second lens which faces the deflecting means in the main scanning direction is defined as R3m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R3s and the radius of curvature of that surface of the second lens which faces the surface to be scanned in the main scanning direction is defined as R4m and the radius of curvature of the aforementioned surface in the sub scanning direction is defined as R4s, the condition that
R1m less than R2m less than 0 less than R4m less than R3m 
R2s less than R1s less than 0 
xe2x80x83R3s less than R4s less than 0
R1m less than R1s less than 0 
R2s less than R2m less than 0 
|R4s| less than R4m 
are satisfied.
In the above-described optical scanning apparatus, the effective end portions of all surfaces of the first and second lenses are displaced more toward the deflecting means than the surface vertexes of the lenses.
In the above-described optical scanning apparatus, the first optical element or the second optical element has a diffraction optical element.
In the above-described optical scanning apparatus, the first optical element is a diffraction optical element, and when the power of that diffracting surface of the diffraction optical element which faces the deflecting means in the main scanning direction is defined as xcfx86d1 and the power of that diffracting surface of the diffraction optical element which faces the surface to be scanned in the main scanning direction is defined as xcfx86d2, the conditions that
xcfx86d1xc3x97xcfx86d2 less than 0 
|xcfx86d2| greater than |xcfx86d1|
are satisfied.
In the above-described optical scanning apparatus, a plurality of beams are emitted from the light source means.
Also, the image forming apparatus of the present invention is an image forming apparatus provided with the above-described optical scanning apparatus, a photosensitive member disposed on the surface to be scanned, a developing device for developing an electrostatic latent image formed on the photosensitive member by the beam scanned by the optical scanning apparatus as a toner image, a transferring device for transferring the developed toner image to a transfer material, and a fixing device for fixing the transferred toner image on the transfer material.
Also, the image forming apparatus of the present invention is an image forming apparatus provided with the above-described optical scanning apparatus, and a printer controller for converting code data inputted from an external device into an image signal and inputting it to the optical scanning apparatus.